1. Field of the Invention
The present invention relates to a wafer of semiconductor material for fabricating integrated devices, and to a process for its fabrication.
2. Discussion of the Related Art
As is known, in the microelectronics industry, the substrate of integrated devices is commonly formed from monocrystalline silicon wafers. In recent years, however, by way of an alternative to all-silicon wafers, so-called SOI (Silicon-on-Insulator) wafers have been proposed. SOI wafers include two layers of silicon, one thinner than the other, separated by a layer of silicon oxide (see, for example, the article entitled: "Silicon-on-Insulator Wafer Bonding-Wafer Thinning Technological Evaluations" by J. Hausman, G. A. Spierings, U. K .P. Bierman and J. A. Pals, Japanese Journal of Applied Physics, Vol. 28, N. 8, Aug. 1989, p. 1426-1443).
FIG. 1 shows a cross section of a portion of a SOI wafer 1 presenting a first (thicker) layer of monocrystalline silicon 2, an intermediate silicon oxide layer 3, and a second (thinner) layer of monocrystalline silicon 4 on which an epitaxial layer (not shown) may subsequently be grown. The intermediate oxide layer preferably has a thickness L.sub.1 of 0.2 to 10 .mu.m.
SOI wafers have recently attracted a good deal of attention on account of the major advantages of integrated circuits with substrates formed from such wafers, as compared with the same circuits with conventional monocrystalline silicon substrates. The advantages may be summed up as follows:
a) higher switching speed; PA1 b) better immunity to noise; PA1 c) low loss currents; PA1 d) no latch-up; PA1 e) reduced stray capacitance; PA1 f) better resistance to radiation; and PA1 g) higher component packing density.
A typical SOI wafer fabrication process will now be described with reference to the FIG. 2 diagram.
The process commences with two normally fabricated monocrystalline silicon wafers 10 and 11. Wafer 10 is the one eventually constituting the bottom base layer, and is accurately sized; while wafer 11 is the one which will eventually be thinned and bonded (via the interposition of an oxide layer) to wafer 10. Wafer 11 (also known as the "bond wafer") is oxidized thermally to form an oxide layer 12 covering it entirely; and, after being cleaned, wafers 10 and 11 are superimposed and heat treated at 1100.degree. C. for roughly 2 hours to "bond" them together.
During bonding, the following chemical reaction takes place: EQU .tbd.SiOH+HOSi.tbd..fwdarw..tbd.Si--O--Si.tbd.+H.sub.2 O (1)
wherein the SiOH groups are present in oxide layer 12; at the heat treatment temperature shown, OH ions are present in oxide layer 12, to form a strong chemical bond between oxide layer 12 (and hence wafer 11) and wafer 10, and so form a semifinished wafer 13.
Semifinished wafer 13 is then surface ground to form a body 14, and finally lapped and polished to form wafer 1.
Despite the major advantages listed above, SOI wafers are of limited application in power circuits on account of the poor thermal conductivity of the silicon oxide layer. Silicon, in fact, is known to present a thermal conductivity of 150 W/(m.degree.C.), as compared with 1.4 W/(m.degree.C.) for silicon oxide (two orders of magnitude lower). As the total thermal resistance of the wafer equals the sum of the thermal resistances of the individual layers (in series with one another), wafer 1, for a given total thickness, presents a much greater thermal resistance as compared with a traditional monocrystalline silicon wafer. In other words, to achieve the same thermal resistance, silicon layer 2 would have to be thinned by a factor depending on the ratio between the conductivity of silicon oxide and monocrystalline silicon (as shown above, equal to roughly 100) and bearing in mind the thickness of the oxide layer. Such thinning, however, would impair the mechanical strength of the wafer, and is therefore unfeasible, at least for the great majority of applications.
In view of the continual reduction in component size and the need for operating at an increasingly high operating current density, SOI substrates therefore fail to provide for sufficient heat dissipation, thus resulting in excessively high junction temperatures, and impaired reliability of the components. In the case of power components, in particular, a limitation in current flow occurs. Such negative effects are especially damaging in the case of high-voltage integrated circuits requiring thicker intermediate oxide layers. Furthermore, in case of power pulses (e.g. in case of electrostatic discharges), heat cannot be dissipated and locally causes fast increases in temperature.
It is an object of the present invention to provide a wafer which exploits the inherent advantages of SOI technology, but which presents none of the above limitations of application.